1. Field of the Invention
The present invention relates to receivers, and more specifically to a receiver for down-converting an inputted signal via an antenna and then demodulating the converted signal.
2. Description of the Background Art
Conventionally, in some cases, a receiver down-converts a signal S inputted from outside to a signal with an intermediate frequency band (hereinafter referred to as IF band). Down-conversion can be realized by mixing the inputted signal S (frequency fS) with a local oscillation output VO (frequency fO) in a mixer inside the receiver.
The local oscillation output VO is conventionally generated by a voltage controlled oscillator (hereinafter referred to as VCO) having a circuitry constitution as shown in FIG. 11. The VCO of FIG. 11 is structured of discrete components, comprising a surface acoustic wave (SAW) resonator 111, a variable capacitance diode 112 and other components. Note that FIG. 11 also shows an equivalent circuit of the SAW resonator 111. The variable capacitance diode 112 is connected in parallel to the SAW resonator 111. With the capacity of the variable capacitance diode 112 adjusted and further with a predetermined control voltage VC applied to the VCO, the VCO generates the local oscillation output VO having variable frequency.
The SAW resonator 111 is expensive and large in size, and its peripheral circuits are structured of discrete components. It is therefore difficult to downsize the VCO and construct it at low cost, and furthermore know-how for mounting these components is required. Against such background, the VCO has come to be constituted on an integrated circuit. FIG. 12 shows a circuit diagram of a VCO constituted on an integrated circuit. As compared with the VCO of FIG. 11, the VCO of FIG. 12 is different in that it takes an integrated circuit form and that an LC oscillator 121 is substituted for the SAW resonator 111.
Assume that a large number of ICs including the VCOs of FIG. 12 are manufactured under certain conditions. For each of the VCOs, a characteristic of the frequency fO of the local oscillation output VO with respect to the control voltage VC (hereinafter referred to as fO vs. VC characteristic) is measured. The fO vs. VC characteristic curve has a linear region and a saturation region. Furthermore, the fO vs. VC characteristic curves disperse with a constant deviation from a design target of the VCOs (refer to a double-headed arrow in FIG. 13). Such dispersion is hereinafter referred to as manufacturing dispersion. Due to this manufacturing dispersion, the saturation region of the fO vs. VC characteristic curve is within a receive band B in some cases. As a result, the receiver cannot down-convert the inputted signal S correctly. It is therefore ideal that each VCO manufactured under the same conditions does not have manufacturing dispersion, which is however difficult in reality. The receive band B is a frequency band which the receiver including the VCOs has to receive and also with which the above signal S is sent out.
For this reason, the following method has been considered. A plurality of VCOs with different fO vs. VC characteristic curves from each other are integrated in an IC. When the control voltage VC of the same level is applied, the plurality of VCOs generate the local oscillation output VO with a different frequency fO, respectively. A VCO control portion is placed on the periphery of each of such VCOs. The VCO control portion has to select one VCO whose fO vs. VC characteristic curve reliably covers the receive band B from among the plurality of VCOs. Further, this selecting processing is preferably carried out at high speed.
Therefore, an object of the present invention is to provide a receiver for selecting an appropriate VCO at high speed. This object is achieved by the following aspects. Further, each aspect has unique technical effects as described below.
A first aspect of the present invention is directed to a receiver for subjecting an inputted signal via an antenna to down-conversion and then demodulating the converted signal, comprising:
a plurality of voltage controlled oscillators (hereinafter referred to as VCOs), each provided with a common control voltage and generating a local oscillation output having a different frequency according to the control voltage;
a PLL circuit for generating the control voltage based on the local oscillation output fed back from each of the VCOs and a reference signal having a reference frequency;
a mixer for mixing frequencies of the inputted signal via the antenna and the local oscillation output from each of the VCOs, and performing the down-conversion; and
a VCO control portion for testing each of the VCOs in a test mode to be executed in advance, and controlling switching of the VCOs in a receive mode of receiving the inputted signal via the antenna;
in the test mode, the VCO control portion
detecting whether the PLL circuit locks using the local oscillation output from each of the VCOs or not as sequentially switching and activating the VCOs, and determining one appropriate VCO based on a detected result;
in the receive mode, the VCO control portion
selectively activating the VCO determined in the test mode, and providing a local oscillation output of the VCO f or the mixer.
In the first aspect, in the test mode, the appropriate VCO is selected for use in the receive mode. In this test mode, the VCO control portion sequentially switches and activates the VCOs to determine an optimal VCO based on the detected result as to whether the PLL circuit locks using the local oscillation output from each VCO. Therefore, in the receive mode, the mixer performs down-conversion using the local oscillation output from the VCO with which the PLL circuit reliably locks. This allows the receiver according to the first aspect to select an appropriate VCO for the above down-conversion.
According to a second aspect, in the first aspect, the VCO control portion holds the detected result in a first table as pattern data, refers to a second table into which the appropriate VCO for each assumed pattern is written, and determines the appropriate VCO corresponding to the pattern data held in the first table.
In the second aspect, the VCO control portion determines the optimal VCO corresponding to the pattern data stored in the first table referring to the second table. Previously written into the second table are assumed patterns and their corresponding optimal VCOs. This allows the receiver according to the second aspect to select the optimal VCO corresponding to the detected result.
According to a third aspect, in the first aspect, the second table is configured based on manufacturing dispersion of the VCOs.
In accordance with the third aspect, the second table is configured based on the above manufacturing dispersion. This allows the receiver according to the third aspect to select the optimal VCO irrespective of manufacturing dispersion of the VCOs.
According to a fourth aspect, in the first aspect, the PLL circuit includes a programmable divider for dividing the fed-back local oscillation output using a predetermined dividing ratio set by the VCO control portion, and generates a control voltage based on a local oscillation output divided by the programmable divider and the reference signal; and
in the test mode, the VCO control portion sets a reference dividing ratio with which each of the VCOs can generate a local oscillation output having a frequency within a band in which the inputted signal is included as the predetermined dividing ratio.
In accordance with the fourth aspect, since the reference dividing ratio as described above is set in the programmable divider, it is possible to let the PLL circuit lock according to the inputted signal.
According to a fifth aspect, in the fourth aspect, the reference dividing ratio is a dividing ratio with which each of the VCOs can generate a local oscillation output having a center frequency of the band.
In the fifth aspect, as evident from the above, the reference dividing ratio is set based on a center frequency of the above band, that is, an average value, and PLL circuit can thus lock at the highest speed. This allows the receiver to execute the test mode in the shortest time.
According to a sixth aspect, in the first aspect, the VCO control portion stores information of the VCO determined in the test mode executed last time; and
when executing the test mode again, the VCO control portion first tests the stored information of the VCO and determines again that the VCO is an optimal VCO when the PLL circuit locks using the local oscillation output from the VCO.
In accordance with the sixth aspect, when it is determined again that the VCO first tested is the optimal one at the time of executing the test mode again, the VCO control portion makes a transition to the receive mode. This can reduce the time for a transition from the test mode to the receive mode.
According to a seventh aspect, in the first aspect, the PLL circuit and each of the VCOs are integrated in a same circuit.
In accordance with the seventh aspect, the above integration in a circuit allows reduction in size and cost of the receiver. Further, unlike when discrete components are used, know-how about mounting the components is not required for manufacturing the receiver.
An eighth aspect is directed to a receiver for subjecting a signal inputted via an antenna to down-conversion and then demodulating the converted signal, comprising:
a plurality of voltage controlled oscillators (hereinafter referred to as VCOs), each provided with common control voltage and generating a local oscillation output having a different frequency according to the control voltage;
a PLL circuit for generating the control voltage based on the local oscillation output fed back from each of the VCOs and a reference signal having a reference frequency;
a mixer for mixing frequencies of the inputted signal via the antenna and the local oscillation output from each of the VCOs, and performing the down-conversion; and
a VCO control portion for testing each of the VCOs in a test mode to be executed in advance, and controlling switching of the VCOs in a receive mode of receiving the signal inputted via the antenna;
in the test mode, the VCO control portion
detecting whether the PLL circuit locks using the local oscillation output from each of the VCOs as sequentially switching and activating the VCOs, and when a value of the control voltage generated by the PLL circuit is within a predetermined range, determining which one of the VCOs provided with the control voltage is an appropriate VCO;
in the receive mode, the VCO control portion
selectively activating the VCO determined in the test mode, and providing a local oscillation output of the VCO for the mixer.
In the eighth aspect, in the test mode, the optimal VCO is selected for use in the receive mode. In this test mode, the VCO control portion sequentially switches and activates the VCOs to determine an optimal VCO based on the detection result as to whether the PLL circuit locks using the local oscillation output from each VCO or not and based on whether the control voltage value which the PLL circuit generates using the local oscillation output is within a predetermined range or not. On detecting the VCO which satisfies these two conditions, the VCO control portion determines that this VCO is the optimal one. Therefore, in some cases, the VCO control portion does not activate all VCOs. This allows the receiver according to the eighth aspect to execute the test mode at higher speed than the receiver according to the first aspect and to make a transition to the receive mode. Furthermore, in the receive mode, the mixer performs down-conversion using the local oscillation output from the VCO with which the PLL circuit reliably locks. This allows the receiver according to the eighth aspect to select an appropriate VCO for the above down-conversion from among the plurality of VCOs.
According to a ninth aspect, in the eighth aspect, the predetermined range includes only one value of the control voltage which is provided when each of the VCOs generates a local oscillation output having a same frequency.
In the ninth aspect, the control portion determines an optimal VCO based on the above predetermined range, and therefore only the optimal VCO can be selected.
According to a tenth aspect, in the eighth aspect, the PLL circuit includes a programmable divider for dividing the fed-back local oscillation output using a predetermined dividing ratio set by the VCO control portion, and generates a control voltage based on a local oscillation output divided by the programmable divider and the reference signal; and
in the test mode, the VCO control portion sets a reference dividing ratio with which each of the VCOs can generate a local oscillation output having a frequency within a band in which the inputted signal is included as the predetermined dividing ratio.
According to an eleventh aspect, in the tenth aspect, the reference dividing ratio is a dividing ratio with which each of the VCOs can generate a local oscillation output having a center frequency of the band.
According to a twelfth aspect, in the eighth aspect, the VCO control portion stores information of the VCO previously determined in the test mode executed last time; and
when executing the test mode again, the VCO control portion first tests the stored information of the VCO and determines again that the VCO is an optimal VCO when the PLL circuit locks using the local oscillation output from the VCO.
According to a thirteenth aspect, in the eighth aspect, the PLL circuit and each of the VCOs are integrated in a same circuit.
These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.